Many pixels vs fewer pixels?

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NikonUser
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Many pixels vs fewer pixels?

Post by NikonUser »

I'm confused.
In a couple of recent posts I interpreted the discussion to mean that an image stack composed of large JPGs (4,288 x 2,848 pixels) shot with a reversed 50/2.8 Nikon El Nikkor enlarging lens set at f/6.7 would show diffraction but if the stack was composed from small JPG's (2,144 x 1,424 px) the diffraction would be significanlty less - (and produce a sharper image ??).

This 7mm fly was shot with a reversed 50/2.8 El Nik @ f/6.7.
1 image was a stack of large fine (=compression ratio of 1:4) JPGs and 1 was a stack of small fine JPGs. Purposely not saying which is which. I think I see a difference, but I'm biased as I know which is which

Images reduced to 800 px, otherwise no processing - straight out of ZS.

Full frame of 11mm, but empty space right and left cropped out. Height was not cropped.
On the 23.6mm wide D90 sensor I calculated the mag. as 2.1x.
Effective f-stop for the large JPGs = 21 ??; for the small f/11 ??

Does this mean the stack from the small JPGs has less diffraction and is a "better" image?
Or at this mag. and at 800px to be viewed on a monitor maybe it makes no difference which JPG size one uses (small, medium, large; in Nikon's terminology)
Image
Image
NU09140 NU09141
Last edited by NikonUser on Sat Oct 10, 2009 1:57 pm, edited 1 time in total.
NU.
student of entomology
Quote – Holmes on ‘Entomology’
” I suppose you are an entomologist ? “
” Not quite so ambitious as that, sir. I should like to put my eyes on the individual entitled to that name.
No man can be truly called an entomologist,
sir; the subject is too vast for any single human intelligence to grasp.”
Oliver Wendell Holmes, Sr
The Poet at the Breakfast Table.

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rjlittlefield
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Re: Many pixels vs fewer pixels?

Post by rjlittlefield »

Does this mean the stack from the small JPGs has less diffraction and is a "better" image?
Or at this mag. and at 800px to be viewed on a monitor maybe it makes no difference which JPG size one uses (small, medium, large; in Nikon's terminology)
It's the second one.

The effective f-number for both images is (2.1+1)*6.7 = 20.77, hence f/21 is a good number. Effective f-number is not affected by pixel count, only by the magnification and the lens setting.

I presume the confusion is caused by my statement HERE that "the pixels in the stacked image [from the small jpegs] are effectively twice as large as we've been assuming, and therefore the effects of diffraction are only half as large."

By "half as large", I meant only that the Airy disk would cover half as many pixels in the "small" images. This is important when we're trying to make sense of pixel-peeps. But when you're looking at images that have been converted to constant size after stacking, then diffraction effects do not depend on the resolution of the images being stacked. Smaller pixel counts mean that the Airy disk will spread over fewer pixels in those intermediate images, but when the result is then scaled to constant size, the Airy disk ends up being the same size in the final image.

There are other effects, however, that do depend on the pixel counts of the images being stacked.

In brief, the more pixels the better. This is because having more numerous but smaller pixels reduces any degradations caused by interpolation. It may also improve the quality of alignment.

One caveat, however, is that if whatever process generates the small images also sharpens them, then that sharpening will propagate right through the stacking software and reappear in the final constant-size output. To really tease apart these effects could require some careful study of the original images, not just looking at final images that have been run through the whole pipeline all the way to posting.

Of the two images that are posted here, layer-and-flash shows that the first image is clearly sharper than the second. Per the discussion above, I'm guessing that the first image comes from large images, the second from small. If that's not the case, then I'd be very interested to get the original stacks so I can figure out what's going on.

By the way, it's clear that this was a fresh specimen. Layer-and-flash shows significant shrinkage of the abdomen and increased bending of the tarsi from the first picture to the second.

--Rik

NikonUser
Posts: 2636
Joined: Thu Sep 04, 2008 2:03 am
Location: southern New Brunswick, Canada

Post by NikonUser »

Thanks Rik, its getting a lot clearer. Actually, I think you told me the same a while back; at the 800px level on a monitor there was nothing to be gained by shooting very high pixel-count images.

The new twist, by my interpretation, was that a high-pixel-count image at a small f-stop would give a poorer image than a low-pixel-count image at the same f-stop.

Yes correct on both accounts, top image "large" and fresh fly. Amazing how quickly they dry and start to shrivel. I do have an air conditioner going full blast as it has been so wet here that I need to dehumidify the air.
NU.
student of entomology
Quote – Holmes on ‘Entomology’
” I suppose you are an entomologist ? “
” Not quite so ambitious as that, sir. I should like to put my eyes on the individual entitled to that name.
No man can be truly called an entomologist,
sir; the subject is too vast for any single human intelligence to grasp.”
Oliver Wendell Holmes, Sr
The Poet at the Breakfast Table.

Nikon camera, lenses and objectives
Olympus microscope and objectives

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